Methods to durably improve properties of silk are known in which epoxides are used. For instance, a method is known in which an epoxy type synthetic resin is padded, dried and cured together with a catalyst selected from amines, acids and acid salts (Japanese Patent Publication No. 1958/10654). However, the envisaged maintenance of whiteness is not so improved. Another method is also known in which an epoxy polymer is padded, dried and steamed or cured using an alkali metal hydroxide or an alkaline salt of an alkali metal as a catalyst (Japanese Patent Publication No. 1963/25198). However, this method is not suitable for practical use, because embrittlement and yellowing of the silk take place easily due to the treatment at high temperatures in the presence of a strongly basic catalyst. A method is also known in which silk is dipped in a solution or emulsion of epoxide and a neutral salt in water or an organic solvent, and then subjected to heat treatment (Japanese Patent Publication No. 1972/24199). However, this method is not suitable for practical use, because a large amount of the neutral salt is required and it is difficult to control the reaction, which leads to the deterioration of silk in many cases. A method is also known in which silk is impregnated with an aqueous solution of a neutral salt, and then heated in a solution of an epoxide in an organic solvent (Japanese Patent Publication No. 1977/38131). However, this method is inferior in economy, because special equipment for environmental pollution prevention, such as a closed system and a recovery system are required, owing to the use of the organic solvent. Further, a method is also known in which silk is dipped in an aqueous solution containing a polyalcohol type epoxy compound and an alkali metal salt of monocarboxylic acid, and then subjected to steaming (Japanese Patent Application Laid-Open No. 1985/81369). However, the modification effects are insufficient. That is, although crease recovery and alkali resistance are fairly improved, the processed product shows only an insufficient prevention of yellowing by sunlight. Another method is known in which silk is given a sprayed or foamed composition containing a neutral salt or weakly basic salt and an epoxide, to which it is then microwave irradiated (Japanese Patent Application Laid-Open 1986/6828). However, the reaction is difficult to control, so that the reproducibility is bad and uniform treatment is difficult to attain without embrittlement.
Further, there is another problem common in all of the above methods. That is, the processed product itself yellows compared to the unprocessed one, so that bleaching is needed.
To lessen such yellowing, a method is known in which silk is impregnated with an aqueous solution of a polyalcohol type glycidyl ether and an alkali metal hydroxide or an alkaline salt of alkali metal, and is allowed to stand (Japanese Patent Applications Laid-Open Nos. 1987/85078 and 1987/85079). However, the attained effect of preventing the yellowing is not sufficient.
In addition, silk fiber has such a disadvantage that abrasion is easily caused by washing. The abrasion resulted from fibrillation of the fibers. No effective method for preventing the fibrillation has been found yet.